Conventionally, in a high power device (a High Electron Mobility Transistor (HEMT) device, as an example), there is a device with a structure in which an InGaN layer of a nitride semiconductor layer is thickly formed between a GaN layer of a nitride semiconductor layer and an AlGaN layer of a nitride semiconductor layer.
But when the InGaN layer is thickly formed, the connection between a lattice of the GaN layer and a lattice of the InGaN layer is cut off, and thereby the InGaN layer is relaxed. When the InGaN layer is relaxed, disturbance in the crystal structure, such as generation of a lattice defect, is generated, and thereby scattering of electrons acting as carriers is generated. For the reason, there was a problem that an electron mobility of a 2 dimensional electron gas (2 Dimensional Electron Gas, 2DEG) in an interface of the InGaN layer and the AlGaN layer is decreased, and an electron density of the 2 dimensional electron gas is decreased.